Foster's sophomore season in the big leagues was one of his best, in which he pitched in 32 games, while starting in 27 of them. He finished with a 14–8 record, and finished second in the American League with an impressive 1.70 ERA. Foster was only behind his Boston Red Sox teammate, Dutch Leonard, who posted a 0.96 ERA, which is now considered the modern day all-time single-season record.

In 1915, Foster posted a 20–8 record, and another impressive 2.11 ERA. Foster most effectively showed his importance to the team in the 1915 World Series where he picked up 2 complete gamewins and only gave up 4 earned runs and struck out 13 batters in 18.0 innings. With the bat, Foster went 4-for-8, with a double and an RBI.

Foster had another good campaign in 1916 acting as a starting pitcher and relief pitcher. He went 14–7 in the season, and posted a decent 3.06 ERA. In the 1916 World Series, Foster came in relief in Game 3, and pitched three scoreless innings. The Red Sox ended up winning the series 4 games to 1, and became the first back-to-back winners of the World Series since the Philadelphia Athletics had done it 5 years earlier.

Foster went back to a mainly starting role in 1917, posting an 8–7 record with a 2.53 ERA.

Before the start of the 1918 season, Foster was traded to the Cincinnati Reds for Dave Shean. Rube Foster refused to report to his new team and so the Red Sox sent cash to the Reds to complete the trade.[1]